JP 2007-269707 A, for example, describes, as a method for producing a methacrylic acid ester, a method comprising thermally decomposing a hydrocarbon selected from propane, propylene, butane, 1-butene, 2-butene, isobutane, isobutene, and butadiene to obtain a decomposed gas in which the total content of propyne and propadiene is 2% by weight or more; subjecting a mixed gas of the decomposed gas thus obtained and a decomposed gas evolving from a plant for thermally decomposing a hydrocarbon having 2 to 10 carbon atoms (common name: ethylene plant) to a separation step shared with the ethylene plant to thereby separate into a mixed solution rich in propyne and propadiene; subjecting the separated mixed solution to extractive distillation to thereby separate into purified propyne and crude propadiene containing propadiene as a main component; and then reacting the thus obtained purified propyne with carbon monoxide and methanol in the presence of a palladium catalyst to thereby produce methyl methacrylate.